Diurnal and Semidiurnal Variations in Global SurfaceWind and Divergence Fields

Aiguo Dai and Clara Deser

J. Geophys. Res., in press, 1999.

Abstract

Diurnal and semidiurnal variations in surface winds and
wind divergence overthe globe (50S-70N) are documented using 3-hourly wind
observationsfrom ~10,000 land stations and COADS marine reports during
1976-1997.A strong diurnal cycle in surface winds is found over
land areas (strongestover high terrain and in summer) with an amplitude of
0.6-1.1 m/s for windspeed and 0.5-0.7 m/s for the zonal and meridional wind
components. Surfacewind speed peaks in the early afternoon over most of
the globe. It is suggestedthat increased downward turbulent mixing of momentum
during the day may bea primary cause for the early afternoon maximum of surface
wind speed.

The diurnal anomalies of surface wind divergence tend
to be out of phasein adjacent regions. In particular, land areas (except
for extreme inlandlocations) exhibit maximum divergence around dawn (0600-0800
LST),while nearby oceanic regions have their maximum divergence
in the evening(1700-1900 LST). Thus, there is evidence for a large-scale
diurnalcirculation in which surface air rises and converges
over the continentsand sinks and diverges over nearby oceans in the afternoon
and earlyevening, and the opposite occurs in the early morning.
Over the tropicalAtlantic and Pacific Oceans (10S-10N), a zonally coherent
pattern ofmaximum convergence (divergence) north (south) of the
equator around0900-1200 LST is generally similar to the latitudinal
profile of the meandaily divergence, indicative of an enhancement of the
local Hadley Cellsaround 1030 LST relative to the daily mean. Another zonally
coherentnorth-south dipole occurs over the North Pacific, with
maximum surface winddivergence (convergence) around 0600-0800 LST in the
subtropics (midlatitudes).This phase pattern correlates with cloud cover over the
two regions and isconsistent with the radiative cooling hypothesis put
forth by Gray and colleagues.Over the United States, surface divergence peaks around
0600 LST in the westand the east and around 2000 LST in the center.